The present invention is related generally to simulcast radio communications systems, and more particulary to a method and apparatus for randomly offsetting the frequencies of each transmitter in a multi-transmitter simulcast radio communications system.
Simulcast radio communications systems typically include a plurality of radio transmitters having the same carrier frequency and each located at different geographical locations for simultaneously transmitting the same information signal to fixed, portable or mobile radios located throughout a large geographical area. The information signal transmitted to the radios can be digital, tone or voice signals. This information signal is modulated onto the carrier signal transmitted by each of the transmitters. In areas midway between two transmitters, commonly referred to as overlap areas, where information signals from both transmitters can be received, the received information signal can be degraded by differences in the transmitter modulation levels, delays or phases, or differences in transmitter frequencies. Furthermore, since radios in the overlap areas can receive two or more transmitter signals having approximately the same amplitude, deep cancellation nulls can occur when the transmitter signals are 180.degree. out of phase with one another. Since both the modulation and frequency of the transmitter signals are highly correlated to minimize audio distortion, the deep cancellation nulls resulting from multi-transmitter interference that occurs in the overlap areas can last for a few seconds or as long as several minutes. Thus, radios located in a null can not receive information signals from the simulcasting transmitters for a long period of time.
The interruption of communications due to deep cancellation nulls of long duration can be alleviated somewhat by intentionally offsetting the frequency of the simulcasting transmitters. For example, the transmitter frequencies may be offset by 10 to 50 Hz relative to one another. Although this transmitter frequency offsetting technique insures that any deep cancellation nulls that do occur are of very short duration, it also creates an audible beat frequency having a repetition rate equal to the frequency difference between the transmitter signals. This beat frequency may be acceptable in some data communications systems, but it is very annoying to the receiving party in voice communications systems. Thus, there is a long felt need for a way of substantially reducing the information signal degradation due to the deep cancellation nulls occurring in the overlap areas between one or more transmitters in simulcast radio communications systems.